The proposed neurophysiological experiments are motivated by two fundamental aspects of sound-level encoding that are strongly affected by cochlear damage and that currently provide significant challenges for the design of effective hearing aids. (1) Loudness recruitment represents the reduced dynamic range and abnormal loudness growth present in almost all hearing-impaired persons with cochlear damage. It is often assumed that loudness is related to the total neural activity evoked by a sound; however, the nature of this relation remains unclear in normal and impaired ears. One aim is to systematically characterize the growth of neural activity with increasing stimulus level in a population of auditory-nerve (AN) fibers for normal and hearing-impaired cats. Stimuli are based on typical psychophysical studies that measure loudness recruitment, and include tones, broadband noise, speech, and masked tones. (2) Robust spectral-shape encoding of complex acoustic stimuli is necessary for many basic auditory functions that underlie communication. A significant enhancement in the representation of speech has been reported in the transformation from the AN to the cochlear nucleus (CN) in normal-hearing animals. The second aim is to compare spectral-shape encoding in the CN of normal and hearing- impaired cats. Two methods will be used to evaluate the relative-level sensitivity for encoding spectral shape in the different cell types of the ventral CN. The data collected will improve the understanding of how cochlear damage affects the neural representation of sound, and will provide useful information for the design of new hearing-aid strategies to overcome loudness recruitment while maintaining and enhancing spectral representation.